If 1 mol N_2(g) reacts with 3 mol H_2 (g) in a flexible container at constant temperature and pressure, would you expect the total gas volume to increase or decrease?

${N}_{2} \left(g\right) + 3 {H}_{2} \left(g\right) \rightarrow 2 N {H}_{3} \left(g\right)$